The episodic nature of the Earth's glacial and interglacial
periods within the present Ice Age (the last couple of million years) have
been caused primarily by cyclical changes in the Earth's circumnavigation of
the Sun. Variations in the Earth's
eccentricity, axial tilt, and precession comprise the three dominant
cycles, collectively known as the
Milankovitch Cycles for Milutin Milankovitch, the Serbian astronomer
and mathematician who is generally credited with calculating their
magnitude. Taken in unison, variations in these three cycles creates
alterations in the seasonality of solar radiation reaching the Earth's
surface. These times of increased or decreased solar radiation directly
influence the Earth's climate system, thus
impacting the advance and retreat of
Earth's glaciers.

It is of primary
importance to explain that climate change, and subsequent periods of
glaciation, resulting from the following three variables is not due to
the total amount of solar energy reaching Earth. The three Milankovitch
Cycles impact the
seasonality and location
of solar energy around the Earth, thus impacting contrasts between
the seasons.

The first of the three Milankovitch Cycles is the Earth's
eccentricity. Eccentricity is, simply, the shape
of the Earth's orbit around the Sun. This constantly fluctuating, orbital
shape ranges between more and less elliptical (0 to 5% ellipticity) on
a cycle of about 100,000 years. These oscillations, from more
elliptic to less elliptic, are of prime importance to glaciation in that it
alters the distance from the Earth to the Sun, thus changing the distance
the Sun's short wave radiation must travel to reach Earth, subsequently
reducing or increasing the amount of radiation received at the Earth's
surface in different seasons.

Today a difference of only about 3 percent occurs between
aphelion (farthest point) and perihelion (closest point). This 3 percent
difference in distance means that Earth experiences a 6 percent increase in
received solar energy in January than in July. This 6 percent range of
variability is not always the case, however. When the Earth's orbit is most
elliptical the amount of solar energy received at the perihelion would be in
the range of 20 to 30 percent more than at aphelion. Most certainly these
continually altering amounts of received solar energy around the globe
result in prominent changes in the Earth's climate and glacial regimes. At
present the orbital eccentricity is nearly at the minimum of its cycle.

Axial tilt, the second of the three Milankovitch Cycles, is
the inclination of the Earth's axis in relation to its plane of orbit around
the Sun. Oscillations in the degree of Earth's axial tilt occur on
a periodicity of 41,000 years from 21.5 to 24.5 degrees.

Today the Earth's axial tilt is about 23.5 degrees, which
largely accounts for our seasons. Because of the periodic variations of
this angle the severity of the Earth's seasons changes. With less axial tilt
the Sun's solar radiation is more evenly distributed between winter and
summer. However, less tilt also increases the difference in radiation
receipts between the equatorial and polar regions.

One hypothesis for Earth's reaction to a smaller degree of
axial tilt is that it would promote the growth of ice sheets. This response
would be due to a warmer winter, in which warmer air would be able to hold
more moisture, and subsequently produce a greater amount of snowfall. In
addition, summer temperatures would be cooler, resulting in less melting of
the winter's accumulation. At present, axial tilt is in the middle of its
range.

The third and final of the Milankovitch Cycles is Earth's
precession. Precession is the Earth's slow wobble as it spins on axis. This
wobbling of the Earth on its axis can be likened to a top running down, and
beginning to wobble back and forth on its axis. The precession of Earth
wobbles from pointing at Polaris (North Star) to pointing at the star Vega.
When this shift to the axis pointing at Vega occurs, Vega would then be
considered the North Star. This top-like wobble, or precession, has
a periodicity of 23,000 years.

Due to this wobble a climatically significant alteration
must take place. When the axis is tilted towards Vega the positions of the
Northern Hemisphere winter and summer solstices will coincide with the
aphelion and perihelion, respectively. This means that the Northern
Hemisphere will experience winter when the Earth is furthest from the Sun
and summer when the Earth is closest to the Sun. This coincidence will
result in greater seasonal contrasts. At present, the Earth is at
perihelion very close to the winter solstice.